cc: Use correct message loop proxy in BlockingTaskRunner

cc/trees/blocking_task_runner.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_BLOCKING_TASK_RUNNER_H_
 #define CC_TREES_BLOCKING_TASK_RUNNER_H_
 
 #include <vector>
 
 #include "base/location.h"
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
 #include "base/synchronization/lock.h"
 #include "base/threading/platform_thread.h"
 #include "cc/base/cc_export.h"
 
 namespace cc {
 
 // This class wraps a SingleThreadTaskRunner but allows posted tasks to be
 // run without a round trip through the message loop. This shortcutting
 // removes guarantees about ordering. Tasks posted while the
 // BlockingTaskRunner is in a capturing state will run in order, and tasks
 // posted while the BlockingTaskRunner is /not/ in a capturing state will
 // run in order, but the two sets of tasks will *not* run in order relative
 // to when they were posted.
 //
 // To use this class, post tasks to the task runner returned by
-// BlockingTaskRunner::current() on the thread you want the tasks to run.
-// Hold a reference to the BlockingTaskRunner as long as you intend to
-// post tasks to it.
+// BlockingTaskRunner::Create() with the SingleThreadTaskRunner for the thread

[danakj, 2014/08/28 17:10:46]

i think this sentence is awkward. period after Create() and describe what STTR
you want to pass as its own sentence.

[Sami, 2014/08/28 18:21:16]

Yah, not sure what I was going for there. Rewritten.

+// you want the tasks to run.
 //
-// Then, on the thread from which the BlockingTaskRunner was created, you
-// may instantiate a BlockingTaskRunner::CapturePostTasks. While this object
+// Then, on the thread that the given task runner belongs to, you may
+// instantiate a BlockingTaskRunner::CapturePostTasks. While this object
 // exists, the task runner will collect any PostTasks called on it, posting
 // tasks to that thread from anywhere. This CapturePostTasks object provides
 // a window in time where tasks can shortcut past the MessageLoop. As soon
 // as the CapturePostTasks object is destroyed (goes out of scope), all
-// tasks that had been posted to the thread during the window will be exectuted
+// tasks that had been posted to the thread during the window will be executed
 // immediately.
 //
 // Beware of re-entrancy, make sure the CapturePostTasks object is destroyed at
 // a time when it makes sense for the embedder to call arbitrary things.
 class CC_EXPORT BlockingTaskRunner
     : public base::RefCountedThreadSafe<BlockingTaskRunner> {
  public:
-  // Returns the BlockingTaskRunner for the current thread, creating one if
-  // necessary.
-  static scoped_refptr<BlockingTaskRunner> current();
+  // Creates a BlockingTaskRunner for a given SingleThreadTaskRunner.
+  // |task_runner| will be used to run the tasks which are posted while we are
+  // not capturing. |task_runner| should belong to same the thread on which
+  // capturing is done.
+  static scoped_refptr<BlockingTaskRunner> Create(
+      scoped_refptr<base::SingleThreadTaskRunner> task_runner);

[danakj, 2014/08/28 17:10:46]

I'm starting to wonder if this class needs to be ref counted at all anymore. Is
there any place that needs to hold a ref now? Can this return a scoped_ptr
instead?

[Sami, 2014/08/28 18:21:17]

Great idea, the ownership is much simpler now so a scoped_ptr should do.

 
   // While an object of this type is held alive on a thread, any tasks
   // posted to the thread will be captured and run as soon as the object
-  // is destroyed, shortcutting past the MessageLoop.
+  // is destroyed, shortcutting past the task runner.
   class CC_EXPORT CapturePostTasks {
    public:
-    CapturePostTasks();
+    explicit CapturePostTasks(
+        scoped_refptr<BlockingTaskRunner> blocking_runner);

[danakj, 2014/08/28 17:10:46]

make this a raw pointer

[Sami, 2014/08/28 18:21:16]

Done.

     ~CapturePostTasks();
 
    private:
     scoped_refptr<BlockingTaskRunner> blocking_runner_;

[danakj, 2014/08/28 17:10:46]

and this

[Sami, 2014/08/28 18:21:16]

Done.

 
     DISALLOW_COPY_AND_ASSIGN(CapturePostTasks);
   };
 
   // True if tasks posted to the BlockingTaskRunner will run on the current
   // thread.
   bool BelongsToCurrentThread();
 
   // Posts a task using the contained SingleThreadTaskRunner unless |capture_|
   // is true. When |capture_| is true, tasks posted will be caught and stored
   // until the capturing stops. At that time the tasks will be run directly
-  // instead of being posted to the SingleThreadTaskRunner.
+  // instead of being posted to the underlying task runner.
   bool PostTask(const tracked_objects::Location& from_here,
                 const base::Closure& task);
 
  private:
   friend class base::RefCountedThreadSafe<BlockingTaskRunner>;
 
   explicit BlockingTaskRunner(
       scoped_refptr<base::SingleThreadTaskRunner> task_runner);
   virtual ~BlockingTaskRunner();
 
   void SetCapture(bool capture);
 
-  base::PlatformThreadId thread_id_;
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
 
   base::Lock lock_;
   int capture_;
   std::vector<base::Closure> captured_tasks_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_BLOCKING_TASK_RUNNER_H_